Tachycardia is an abnormal heart rhythm characterized by rapid activation of one or more chambers of the heart of a patient. Tachycardia is often qualified by the locus of origin: a tachycardia that originates in the ventricles of the heart is called a ventricular tachycardia (VT) and a tachycardia that originates in the atria of the heart is called an atrial tachycardia (AT) or a supraventricular tachycardia (SVT). Some VTs, if untreated, may accelerate into ventricular fibrillation (VF), in which the pumping ability of the heart is seriously impaired.
There are many therapies that may be applied to treat tachycardia. Some tachycardias respond well to medication, and others may be treated with surgery such as radio frequency (RF) ablation. In some patients, VT or AT may respond well to antitachycardia pacing (ATP), in which small electric stimulations from an implantable pulse generator (IPG) in an implantable medical device (IMD) disrupt the propagation of electrical signals that cause the tachycardia.
In some circumstances, however, a tachycardia may fail to terminate in response to therapies such as these. Some IMDs may therefore include the capability of delivering a higher energy cardioversion shock to terminate the tachycardia. Cardioversion is an effective therapy in treating well organized single loop tachycardias. Application of a cardioversion shock at a particular moment depolarizes cardiac tissue to prevent re-entry, thereby terminating the tachycardia.
In conventional cardioversion therapy, an IMD delivers a cardioversion shock synchronized to a cardiac event, such as an R-wave that accompanies a ventricular depolarization. In other words, the IMD delivers the cardioversion shock at a fixed time in relation to the event. The IMD may, for example, time the delivery of a cardioversion shock promptly upon detection of an R-wave.
An external device, such as an external defibrillator, likewise may be capable of applying cardioversion therapy. Like implanted devices, external devices may sense cardiac events and may apply cardioversion shocks synchronized to the cardiac events.